Many first report peanut allergy symptoms in adulthood, study finds By Alexandria Hein At least 4.5 million adults in the U.S. are impacted by a peanut allergy, according to researchers. EVANSTON, Ill. - While a peanut allergy is commonly associated with pediatric care, a new study has found that up to one in six adults with sensitivity to peanuts developed it after age 18. The report, put forth by Northwestern University researchers in the Journal of Allergy and Clinical Immunology, found that 2.9% of U.S. adults report a current peanut allergy, but only 1.8% report a physician-diagnosed allergy and or history of reaction symptoms.